Transplantation of hematopoietic stem and progenitor cells from umbilical-cord blood (CB) can restore the function of bone marrow and sustain hematopoietic recovery in both related and unrelated recipients (Gluckman, et al. 1989; Kutzberg, et al. 1996). For patients for whom no suitable related donor is available, this source of hematopoietic stem cells offers substantial advantages, because of the relative ease of procurement; the absence of risk to the donor; the small likelihood of transmitting clinically important infectious diseases such as cytomegalovirus (CMV) and Epstein-Barr (EBV) virus infections; the rapid availability of placental blood to transplantation centres; and the low risk of severe graft-versus-host disease (GVHD) when compared to unrelated bone marrow transplantation (Gluckman, et al. 1997; Rubinstein, et al. 1998). The reduced risk and severity of GVHD permits the use of transplants from HLA-mismatched CBs and improves the likelihood of finding transplant units for patients with uncommon tissue types. For these reasons, worldwide cord-blood banks have increased the use of cord-blood transplantation in patients with hematologic disorders (Silberstein, et al. 1996).
Large retrospective studies of the performed bone marrow transplantations until 1998 with more than 600 evaluable cases have shown that the number of nucleated cells infused per kilogram was a major factor predicting the recovery to clinically relevant neutrophil and platelet counts. The leukocyte content of the graft relates principally to the speed and overall success of engraftment and secondarily to transplantation-related events and event-free survival. Consequently, doses of leukocytes from larger placental-blood collections may improve event-free survival, particularly for older patients (Gluckman, et al. 1997; Rubinstein et al. 1998). For these reasons, the success of cord blood transplants critically depends on the correct determination of the white blood cells WBC counts in CB transplantation units.
It is well established with new-born patients that nucleated red blood cells (NRBC) and lysis resistant red cells are negatively influencing the quality of automated WBC determination in cord blood samples (Cranendonk, et al. 1985; Johannessen, et al. 1990; Hanion, et al. 1997; Walka, et al. 1998).
Analysis of neonatal or cord blood samples has always been different than adult blood samples (Johannessen, et al. 1990). Interpretation of the automated (WBC) count results can be problematical because of the marked alterations in reference limits during the first hours of life (Johannessen, et al. 1990). Newborn blood samples often contain remarkable number of normoblasts sometimes even exceeding WBC counts, which are counted by all haematological analysers based on the Coulter principle as WBCs, namely lymphocytes (Nikischin, et al. 1997; Oski, et al. 1966; Dörner, et al. 1995; Lim, 1994). Further, subpopulations of neonatal red blood cells (RBC) have a higher osmotic resistance than normal adult RBCs (Serrani, et al. 1989).